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Toughness is the ability to absorb energy without fracture. (I called Larrin and gave me this definition) In use knives that are too soft will roll at the edge rendering them useless, and knives that are too hard will chip out at the edge or worse break all together and become useless. Proper heat treating is the best way to ensure the best balance of hardness and toughness.

Good toughness in properly heat treated steels will affect other things like edge stability, edge holding, and the sharpenability of the knife. To increase both the toughness and hardness of the steel correct forging and heat treating cycles must be used.

The amount of carbide in the steel and the size of those carbides along with the over all hardness of the steel has the biggest effect on toughness. Some elements added to steels will increase toughness.

Grains in steel are like soap bubbles in a jar. Grains are different than carbides. The smaller the grain and the smaller the carbides in the steel the tougher the steel will be. Correct forging and heat treating of the steel allows proper grain refinement and to a lesser extent smaller carbide size.

Powder metalurgy steels were developed to be able to increase both wear resistance and toughness by reducing both the grain size and the carbide size and by more even distribution of the carbides in the steel.

Edge stablity, this is the ability for a knife to hold a very fine edge. A fine edge is one that is sharpened to a small angle and to a high grit.

Just like toughness edge stablility is affected most by grain size and carbide size and distribution. Steels with larger carbides and greater volume of carbides are not as stable as ones with smaller and fewer carbides. The smaller the grain the better the edge stability. Pm steels were made to improve edge stability.

There are 2 types of cutting, namely push cutting and slice cutting. Push cutting benefits from finer edges more than slice cutting. Too fine an edge will not work well slice cutting fiberous materials.

The thing that happens to edges that are not stable is that the larger carbides pull out from the edge while in service leaving a flat spot or a hole. This is called carbide pull out.

Hmmm.... kind of casually mentions it in passing. In the context of polishing a flat surface, not really the same as a knife edge.

Also, the context I have always read is the "large" carbides pulling out of the knife edge is the pull out is noticeable. How big is a "large" carbide? I still have my doubts.

What I'd like to see is some photomicrographs of a knife edge showing the missing carbides. Actually, I would love to see ANY photomicrographs of some knife edges, especially a transverse view. Are you aware of any Larrin?